Highly efficient folding of multi-disulfide proteins in superoxidizingEscherichia colicytoplasm

Use of the redox-dependent intein system for enhancing production of the cyclic green fluorescent protein

To expand the reported redox-dependent intein system application, in this work, we used the split intein variant with highly trans-splicing efficiency and minimal extein dependence to cyclize the green fluorescent protein variant reporter in vitro. The CPG residues were introduced adjacent to the intein's catalytic cysteine for reversible formation of a disulfide bond to retard the trans-splicing reaction under the oxidative environment. The cyclized reporter protein in Escherichia coli cells was easily prepared by organic extraction and identified by the exopeptidase digestion. The amounts of extracted cyclized protein reporter in BL21 (DE3) cells were higher than those in hyperoxic SHuffle T7 coexpression system for facilitating the disulfide bond formation. The double His6-tagged precursor was purified for in vitro cyclization of the protein for 3 h. Compared with the purified linear counterpart, the cyclic reporter showed about twofold increase in fluorescence intensity, exhibited thermal and hydrolytic stability, and displayed better folding efficiency in BL21 (DE3) cells at the elevated temperature. Taken together, the developed redox-dependent intein system will be used for producing other cyclic disulfide-free proteins. The cyclic reporter is a potential candidate applied in certain thermophilic aerobes.

Strategies for efficient production of recombinant proteins in Escherichia coli: alleviating the host burden and enhancing protein activity

Escherichia coli, one of the most efficient expression hosts for recombinant proteins (RPs), is widely used in chemical, medical, food and other industries. However, conventional expression strains are unable to effectively express proteins with complex structures or toxicity. The key to solving this problem is to alleviate the host burden associated with protein overproduction and to enhance the ability to accurately fold and modify RPs at high expression levels. Here, we summarize the recently developed optimization strategies for the high-level production of RPs from the two aspects of host burden and protein activity. The aim is to maximize the ability of researchers to quickly select an appropriate optimization strategy for improving the production of RPs.

Construction, Investigation and Application of TEV Protease Variants with Improved Oxidative Stability

Tobacco etch virus protease (TEVp) is a useful tool for removing fusion tags, but wild-type TEVp is less stable under oxidized redox state. In this work, we introduced and combined C19S, C110S and C130S into TEVp variants containing T17S, L56V, N68D, I77V and S135G to improve protein solubility, and S219V to inhibit self-proteolysis. The solubility and cleavage activity of the constructed variants in Escherichia coli strains including BL21(DE3), BL21(DE3)pLys, Rossetta(DE3) and Origami(DE3) under the same induction conditions were analyzed and compared. The desirable soluble amounts, activity, and oxidative stability were identified to be reluctantly favored in the TEVp. Unlike C19S, C110S and C130S hardly impacted on decreasing protein solubility in the BL21(DE3), but they contributed to improved tolerance to the oxidative redox state in vivo and in vitro. After two fusion proteins were cleaved by purified TEVp protein containing double mutations under the oxidized redox state, the refolded disulfide-rich bovine enterokinase catalytic domain or maize peroxidase with enhanced yields were released from the regenerated amorphous cellulose via affinity absorption of the cellulose-binding module as the affinity tag.

In Vivo Imaging with Genetically Encoded Redox Biosensors

Redox reactions are of high fundamental and practical interest since they are involved in both normal physiology and the pathogenesis of various diseases. However, this area of research has always been a relatively problematic field in the context of analytical approaches, mostly because of the unstable nature of the compounds that are measured. Genetically encoded sensors allow for the registration of highly reactive molecules in real-time mode and, therefore, they began a new era in redox biology. Their strongest points manifest most brightly in in vivo experiments and pave the way for the non-invasive investigation of biochemical pathways that proceed in organisms from different systematic groups. In the first part of the review, we briefly describe the redox sensors that were used in vivo as well as summarize the model systems to which they were applied. Next, we thoroughly discuss the biological results obtained in these studies in regard to animals, plants, as well as unicellular eukaryotes and prokaryotes. We hope that this work reflects the amazing power of this technology and can serve as a useful guide for biologists and chemists who work in the field of redox processes.

Applications of catalyzed cytoplasmic disulfide bond formation

Disulfide bond formation is an essential post-translational modification required for many proteins to attain their native, functional structure. The formation of disulfide bonds, otherwise known as oxidative protein folding, occurs in the endoplasmic reticulum and mitochondrial inter-membrane space in eukaryotes and the periplasm of prokaryotes. While there are differences in the molecular mechanisms of oxidative folding in different compartments, it can essentially be broken down into two steps, disulfide formation and disulfide isomerization. For both steps, catalysts exist in all compartments where native disulfide bond formation occurs. Due to the importance of disulfide bonds for a plethora of proteins, considerable effort has been made to generate cell factories which can make them more efficiently and cheaper. Recently synthetic biology has been used to transfer catalysts of native disulfide bond formation into the cytoplasm of prokaryotes such as Escherichia coli. While these engineered systems cannot yet rival natural systems in the range and complexity of disulfide-bonded proteins that can be made, a growing range of proteins have been made successfully and yields of homogenously folded eukaryotic proteins exceeding g/l yields have been obtained. This review will briefly give an overview of such systems, the uses reported to date and areas of future potential development, including combining with engineered systems for cytoplasmic glycosylation.

Process Intensification for an Insect Antimicrobial Peptide Elastin-Like Polypeptide Fusion Produced in Redox-Engineered Escherichia coli

Peptides and proteins containing disulfide bonds can be produced in Escherichia coli by targeting the oxidizing periplasm, co-expressing isomerases or chaperons, refolding from inclusion bodies, or by using redox-engineered E. coli strains. Thus far, protein expression in glutathione reductase and thioredoxin reductase deficient (Δgor ΔtrxB) E. coli strains has required a complex medium. However, a chemically defined medium suitable for large-scale production would be preferable for industrial applications. Recently, we developed a minimal medium supplemented with iron (M9i) for high-density cultivation using E. coli Rosetta gami B(DE3)pLysS cells. Here we show that M9i is suitable for the production of insect metalloproteinase inhibitor (IMPI), which contains five disulfide bonds, in the same E. coli strain. We demonstrated the scalability of the new fed-batch process by combining the scale-up criteria of constant dissolved oxygen (DO) and matching volumetric power inputs (P/V) at the borders of the stirrer cascade. Process intensification was achieved by investigating production feed rates and different induction times. We improved product titers by ~200-fold compared to the standard process in complex medium while maintaining the activity of the IMPI protein. Our results show for the first time that it is possible to produce active proteins containing multiple disulfide bonds in a Δgor ΔtrxB E. coli strain using M9i medium. The success of scale-up and process intensification shows that the industrial production of complex recombinant proteins in such strains using chemically defined M9i minimal medium is feasible.

Persulfide Dioxygenase From Acidithiobacillus caldus: Variable Roles of Cysteine Residues and Hydrogen Bond Networks of the Active Site

Persulfide dioxygenases (PDOs) are abundant in Bacteria and also crucial for H2S detoxification in mitochondria. One of the two pdo-genes of the acidophilic bacterium Acidithiobacillus caldus was expressed in Escherichia coli. The protein (AcPDO) had 0.77 ± 0.1 Fe/subunit and an average specific sulfite formation activity of 111.5 U/mg protein (Vmax) at 40°C and pH 7.5 with sulfur and GSH following Michaelis-Menten kinetics. KM for GSH and Kcat were 0.5 mM and 181 s-1, respectively. Glutathione persulfide (GSSH) as substrate gave a sigmoidal curve with a Vmax of 122.3 U/mg protein, a Kcat of 198 s-1 and a Hill coefficient of 2.3 ± 0.22 suggesting positive cooperativity. Gel permeation chromatography and non-denaturing gels showed mostly tetramers. The temperature optimum was 40-45°C, the melting point 63 ± 1.3°C in thermal unfolding experiments, whereas low activity was measurable up to 95°C. Site-directed mutagenesis showed that residues located in the predicted GSH/GSSH binding site and in the central hydrogen bond networks including the iron ligands are essential for activity. Among these, the R139A, D141A, and H171A variants were inactive concomitant to a decrease of their melting points by 3-8 K. Other variants were inactivated without significant melting point change. Two out of five cysteines are likewise essential, both of which lie presumably in close proximity at the surface of the protein (C87 and C224). MalPEG labeling experiments suggests that they form a disulfide bridge. The reducing agent Tris(2-carboxyethyl)phosphine was inhibitory besides N-ethylmaleimide and iodoacetamide suggesting an involvement of cysteines and the disulfide in catalysis and/or protein stabilization. Mass spectrometry revealed modification of C87, C137, and C224 by 305 mass units equivalent to GSH after incubation with GSSH and with GSH in case of the C87A and C224A variants. The results of this study suggest that disulfide formation between the two essential surface-exposed cysteines and Cys-S-glutathionylation serve as a protective mechanism against uncontrolled thiol oxidation and the associated loss of enzyme activity.

Monitoring cellular redox state under hypoxia using a fluorescent sensor based on eel fluorescent protein

Genetically encoded fluorescent sensors are widely used to visualize secondary messengers, metabolites and dynamic events in living cells. However, almost all of these sensors are based on Aequorea GFPs or GFP-like proteins, which do not correctly maturate and fluoresce under hypoxia or anoxic conditions, greatly limiting their application in biomedical research. Herein, we provide a novel strategy for design of sensors and report a series of thiol redox-sensitive sensor based on a recently discovered oxygen-independent fluorescent protein UnaG from Japanese eel. These redox sensors have large dynamic range, rapid responsiveness, a flexible "switch", and pH-independence, are particularly compatible with hypoxia conditions, and therefore represent a substantial improvement for live-cell redox measurement. We further demonstrated the versatility of these redox sensors, by simultaneously monitoring redox changes and hypoxia state in living cells, thereby proving its capability as a powerful and flexible tool for indexing multidimensional metabolism data in the context of physiological stressors and pathological states. These redox sensors are not only the first case of UnaG-based functional sensors, but also the first case of functional sensors based on non GFP-like proteins. Based on this strategy, more oxygen-independent biosensors could be developed, hence, provide new opportunities for bioimaging.

Direct structural evidence of protein redox regulation obtained by in-cell NMR

The redox properties of cellular environments are critical to many functional processes, and are strictly controlled in all living organisms. The glutathione-glutathione disulfide (GSH-GSSG) couple is the most abundant intracellular redox couple. A GSH redox potential can be calculated for each cellular compartment, which reflects the redox properties of that environment. This redox potential is often used to predict the redox state of a disulfide-containing protein, based on thermodynamic considerations. However, thiol-disulfide exchange reactions are often catalyzed by specific partners, and the distribution of the redox states of a protein may not correspond to the thermodynamic equilibrium with the GSH pool. Ideally, the protein redox state should be measured directly, bypassing the need to extrapolate from the GSH. Here, by in-cell NMR, we directly observe the redox state of three human proteins, Cox17, Mia40 and SOD1, in the cytoplasm of human and bacterial cells. We compare the observed distributions of redox states with those predicted by the GSH redox potential, and our results partially agree with the predictions. Discrepancies likely arise from the fact that the redox state of SOD1 is controlled by a specific partner, its copper chaperone (CCS), in a pathway which is not linked to the GSH redox potential. In principle, in-cell NMR allows determining whether redox proteins are at the equilibrium with GSH, or they are kinetically regulated. Such approach does not need assumptions on the redox potential of the environment, and provides a way to characterize each redox-regulating pathway separately.